Method for forming gate having metal layer in semiconductor device

ABSTRACT

Disclosed are a semiconductor device with a metal gate and a method of manufacturing the same. The method of the present invention includes: preparing a semiconductor substrate having a isolation layer to define an active region; forming a gate insulation layer on the semiconductor substrate; sequentially forming a polysilicon layer, a first metal silicide layer, a metal nitride layer and a metal layer on the gate insulation layer including the isolation layer; etching the metal layer and the metal nitride layer so that the metal layer and the metal nitride layer have a narrower width than that of a desired gate; forming a second metal silicide layer on the first metal silicide layer including the etched metal nitride layer and the metal layer; forming a hard mask on the second metal silicide layer so that the hard mask has a desired gate width; and etching the second metal silicide layer, the first metal silicide layer, the polysilicon layer and the gate insulation layer by using the hard mask as an etching barrier, so as to form a metal gate with a structure in which the metal nitride and the metal layer are enclosed with the first and second metal silicide layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing asemiconductor device, and more particularly to a semiconductor devicewhich includes a metal gate having a low resistance and a method ofmanufacturing the same.

2. Description of the Prior Art

Recently, as a design rule for Metal Oxide Semiconductor Field-EffectTransistor (MOSFET) devices have been rapidly decreased to a level lessthan a sub-100 nm class, RC delay of a gate, i.e. a word-line, hasbecome an important issue. Therefore, in order to solve the RC delay ofthe word-line, application of gate material with a lower resistivity hasbeen attempted as an alternative plane. Specifically, a metal gatestructure formed from stacked layers of a polysilicon layer and a metallayer is used as the gate material, instead of a polysilicide gatestructure formed from stacked layers of a polysilicon layer and a metalsilicide layer. Recently, for example, the application of tungsten gatein which tungsten is used as a metal layer has been actively researched.

Further, as a channel length becomes shorter due to the high integrationof the MOSFET devices, the occurrence of defect in the MOSFET devices isincreasing due to the short channel effect. A recess gate in which arecess is formed as a gate on a portion of a semiconductor substrate inmanufacturing a transistor is being actively researched and developed.According to such a recess gate structure, the channel length can beincreased by forming the gate in the recess, thereby remarkablydecreasing the occurrence of defects due to the short channel effect, incomparison with the conventional planar gate structure.

Hereinafter, a method of manufacturing a semiconductor device having atungsten gate, which is currently used, will be described in brief withreference to FIGS. 1A and 1B.

Referring to FIG. 1A, a isolation layer 2 defining an active region isformed on a semiconductor substrate 1. Next, after a gate insulationlayer 3 is formed in the active region defined by the isolation layer 2in a gate oxidation process, a polysilicon layer 4, a tungsten nitridelayer 5 and a tungsten layer 6 are sequentially formed as a gateconductive layer on the gate insulation layer 3 including the isolationlayer 2.

Referring to FIG. 1B, after a nitride layer is deposited on the tungstenlayer 6, the nitride layer is patterned so as to form a gate hard mask 7defining a gate region. Then, the tungsten layer 6, the tungsten nitridelayer 5, the polysilicon layer 4 and the gate insulation layer 3 aresequentially etched by using the gate hard mask 7 as an etching barrier,so as to form a tungsten gate 8.

Although not shown, a gate re-oxidation step is performed for theresultant of the semiconductor device on which the tungsten gate 8 isformed, thereby recovering etching damages. Next, a series of knownsucceeding steps of forming a Lightly Doped Drain region (LDD region),forming a gate spacer, and forming a junction region are sequentiallycarried out so as to manufacture the semiconductor device with thetungsten gate.

However, the conventional method of manufacturing the semiconductordevice with the tungsten gate as described above has problems asfollows:

First, the gate re-oxidation process is conventionally performed inorder to recover the etching damages, after etching the gate. At thistime, in the case of performing the typical oxidation process, sinceabnormal oxidation may be caused at a side surface of the tungstenoxidation layer, a selective oxidation process in which the tungstenlayer is not oxidized is carried out instead of the typical oxidationprocess. Although the selective oxidation process can prevent occurrenceof the abnormal oxidation phenomenon at the side surface of the tungstenlayer, it cannot prevent the occurrence of defects caused by permeationof oxygen through an interface between the tungsten layer and apolysilicon layer.

Further, in the case of the existing tungsten silicide gate, nodeterioration of the characteristics of the gate caused by stress of thenitride layer for the hard mask after etching the gate has been found.However, in the case of the tungsten gate, a stress induced leakagecurrent and an interface trap density caused by the stress of thenitride layer for the hard mask are increasing. These phenomena occurwhen the stress of the nitride layer for the hard mask is transferred tothe tungsten layer, because the tungsten makes a direct effect on thegate insulation layer in the state that the stress applied to thetungsten is insufficiently relaxed due to its characteristic. As aresult, voids are generated between the gate insulation layer and thepolysilicon layer. To the end, the voids cause the deterioration of thecharacteristics of the gate.

Therefore, in order to develop the high speed device products, it ispossible to realize the semiconductor device to which the tungsten gateis applied so that the aforementioned two problems must be solved.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been developed in order to solvethe above-mentioned problems occurring in the prior art, and an objectof the present invention is to provide a semiconductor device and amethod of manufacturing the same, which can prevent a tungsten layerfrom being abnormally oxidized in a gate re-oxidation process.

Another object of the present invention is to provide a semiconductordevice and a method of manufacturing the same, which can prevent thedeterioration of the characteristics of a tungsten gate caused by stressof a nitride layer for a hard mask.

Still another object of the present invention is to provide asemiconductor device and a method of manufacturing the same, which canprevent the deterioration of the characteristics of a tungsten layer dueto the abnormal oxidation of the tungsten layer and a stress of anitride layer for a hard mask.

According to an aspect of the present invention to accomplish theseobjects, there is provided a semiconductor device which comprises: asilicon substrate; a isolation layer formed on the silicon substrate fordefining an active region; a metal gate which is formed in the activeregion defined by the isolation layer, and enclosed with a metalsilicide layer; and a junction region formed at both sides of the metalgate in the active region on the semiconductor substrate.

Here, the metal gate is formed in such a manner that a gate insulationlayer, a polysilicon layer, a first metal silicide layer, a metalnitride layer and a metal layer are sequentially stacked in a pattern,that a second metal silicide layer is formed on the upper surface of themetal layer and a side of the metal layer including the metal nitridelayer, and that a hard mask is formed in a pattern on the second metalsilicide layer.

The first and second metal silicide layers include a tungsten silicidelayer, the metal nitride layer includes a tungsten nitride layer, themetal layer includes a tungsten layer, and the hard mask includes anitride layer.

The metal nitride layer and the metal layer, which include the secondmetal silicide layer formed on the sides thereof, are formed with awidth identical with or smaller than that of the first metal silicidelayer.

In the semiconductor device of the present invention, the semiconductorsubstrate is recessed the active region in which the metal gate isformed, so as to form a recess gate structure.

In the semiconductor device of the present invention, further, thesemiconductor substrate has a step-gated asymmetry recess structure inwhich both sides of the active region are recessed along a length of theactive region so that a gate region is stepped.

In order to accomplish the objects of the present invention, accordingto another aspect of the present invention, there is provided a methodof manufacturing a semiconductor device, which comprises the steps of:preparing a semiconductor substrate having a isolation layer to definean active region; forming a gate insulation layer on the semiconductorsubstrate; sequentially forming a polysilicon layer, a first metalsilicide layer, a metal nitride layer and a metal layer on the gateinsulation layer including the isolation layer; etching the metal layerand the metal nitride layer so that the metal layer and the metalnitride layer have a narrower width than that of a desired gate; forminga second metal silicide layer on the first metal silicide layerincluding the etched metal nitride layer and the metal layer; forming ahard mask on the second metal silicide layer so that the hard mask has adesired gate width; and etching the second metal silicide layer, thefirst metal silicide layer, the polysilicon layer and the gateinsulation layer by using the hard mask as an etching barrier, so as toform a metal gate with a structure in which the metal nitride and themetal layer are enclosed with the first and second metal silicidelayers.

In order to accomplish the objects of the present invention, there isprovided a method of manufacturing a semiconductor device, whichcomprises the steps of: preparing a semiconductor substrate having aisolation layer to define an active region; etching a gate region in theactive region on the semiconductor substrate, so as to form recesses;forming a gate insulation layer on the semiconductor substrate havingthe recesses; forming a polysilicon layer on the gate insulation layerso as to fill up the recesses; sequentially forming a first metalsilicide layer, a metal nitride layer and a metal layer on thepolysilicon layer; etching the metal layer and the metal nitride layerto have a narrower width than that of a desired gate; forming a secondmetal silicide layer on the first metal silicide layer including theetched metal nitride layer and the metal layer; forming a hard mask onthe second metal silicide layer so that the hard mask has a desired gatewidth; and etching the second metal silicide layer, the first metalsilicide layer, the polysilicone layer and the gate insulation layer byusing the hard mask as an etching barrier so as to form a metal gatewith a structure in which the etched metal nitride layer and the metallayer are enclosed with the first and second metal silicide layer.

In order to accomplish the objects of the present invention, there is aprovided a method of manufacturing a semiconductor device, whichcomprises the steps of: preparing a semiconductor substrate having aisolation layer to define an active region; recessing the both sides ofthe active region on the semiconductor substrate so that a gate regionis stepped in a lengthwise direction of the active region; forming agate insulation layer on the stepped semiconductor substrate; forming apolysilicon layer, a first metal silicide layer, a metal nitride layerand a metal layer on the gate insulation layer; etching the metal layerand the metal nitride layer having a narrower width than that of adesired gate; forming a second metal silicide layer on the first metalsilicide layer including the etched metal nitride layer and the metallayer; forming a hard mask on the second metal silicide layer so thatthe hard mask has a desired gate width; and etching the second metalsilicide layer, the first metal silicide layer, the polysilicon layerand the gate insulation layer by using the hard mask as an etchingbarrier, so as to form a metal gate, which has a structure in that theetched metal nitride layer and the metal layer are enclosed with thefirst and second metal silicide layers, in the active region on thestepped semiconductor substrate.

According to the present invention, the method of manufacturing asemiconductor device further comprises a step of forming a junctionregion at both sides of the metal gate on the semiconductor substrate,after the step of forming the metal gate.

According to the present invention, the first and second metal silicidelayers are formed with tungsten silicide, the metal nitride is formedwith tungsten nitride, the metal layer is formed with tungsten, and thehard mask is formed with nitride. Here, the first metal silicide layerhas a thickness of 200˜400 Å, the metal nitride layer has a thickness of50˜100 Å, the metal layer has a thickness of 300˜400 Å, and the secondmetal silicide layer has a thickness of 200˜400 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B are sectional views illustrating a method ofmanufacturing a conventional semiconductor device having a tungstengate, in which the semiconductor device is shown in sections step bystep;

FIGS. 2A to 2D are sectional views illustrating a method ofmanufacturing a semiconductor device having a metal gate according to anembodiment of the present invention, in which the semiconductor deviceis shown step by step;

FIGS. 3A to 3E are sectional views illustrating a method ofmanufacturing a semiconductor device having a metal gate according toanother embodiment of the present invention, in which the semiconductordevice is shown step by step; and

FIGS. 4A to 4B are sectional views illustrating a method ofmanufacturing a semiconductor device having a metal gate according tostill another embodiment of the present invention, in which thesemiconductor device is shown step by step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIGS. 2A to 2D are sectional views illustrating a method ofmanufacturing a semiconductor device having a metal gate according to anembodiment of the present invention, in which the semiconductor deviceis shown step by step.

Referring to FIG. 2A, a semiconductor substrate 21 is prepared, whichincludes a isolation layer 22 defining an active region. Next, well ioninjection and channel ion injection are sequentially performed. Then,after a gate insulation layer 23, i.e. oxidation layer, is formed on theactive region of the substrate 21 in such a gate oxidization process, adoped poly-silicon layer 24 is formed on the gate oxidation layer 23including the isolation layer 22.

Next, a first metal silicide layer, preferably a first tungsten silicidelayer 25, is formed on the poly-silicon layer 24. Then, after a metalnitride layer, preferably a tungsten nitride layer 26, is deposited, ametal layer, preferably a tungsten layer 27, is deposited on thetungsten nitride layer 26. Here, the first tungsten silicide layer 25has a thickness of 200˜400 Å, and the tungsten nitride layer 26 isformed with a thickness of 50˜100 Å, and the tungsten layer 27 has athickness of 300˜400 Å.

Referring to FIG. 2B, the tungsten layer 27 and the tungsten nitridelayer 26 are etched using a known process. At this time, the etching ofthe tungsten layer 27 and the tungsten nitride layer 26 is performed insuch a manner that a CD bias is increased to the maximum, so that theetched tungsten layer 27 and the tungsten nitride layer 26 have anarrower width than that of a desired gate.

Referring to FIG. 2C, a second metal silicide layer, preferably a secondtungsten silicide layer 28, is formed on the first tungsten silicidelayer 25 including the etched tungsten nitride layer 26 and the tungstenlayer 27. At this time, the second tungsten silicide layer 28 has athickness of 200˜400 Å.

Referring to FIG. 2D, after a nitride layer for a hard mask is formed onthe second tungsten silicide layer 28, the nitride layer is patternedusing a known process so as to form the hard mask 29 having a desiredgate width. Next, the tungsten silicide layer 28, the first tungstensilicide layer 25, the poly-silicon layer 24 and the gate insulationlayer 23, which are located below the hard mask 29, are sequentiallyetched by using the hard mask 29 as an etching barrier, thereby forminga metal gate, i.e. a tungsten gate 30, in which the tungsten nitridelayer 26 and the tungsten layer 27 are enclosed with the first andsecond tungsten silicide layers 25 and 28.

Then, a gate re-oxidation process is performed for the resultantsemiconductor substrate having the tungsten gate 30 formed thereon, inorder to recover an etching damage caused by the etching of the gate.

According to the present invention, here, since the tungsten gate 30 isformed with a core type structure in that the tungsten layer 27including the tungsten nitride layer 26 is enclosed with the first andsecond tungsten silicide layers 25 and 28, the abnormal oxidizationphenomenon is prevented from occurring at the side of the tungsten layer27. In addition, oxygen is completely prevented from penetrating theinterface between the tungsten layer 27 and the poly-silicon layer 24,thereby removing the instability of the interface.

In the tungsten gate structure according to the present invention,moreover, since the second tungsten silicide layer 28 deposited on thesurface of the tungsten layer 27 plays the role of a buffer layer duringthe forming of the nitride layer used for the hard mask, the stress canbe relieved, or prevented from being transmitted to the tungsten layer27 during the forming of the nitride layer for the hard mask. Thus, itis possible to effectively restrain the deterioration of thecharacteristics of the gate which may be caused by the stress of thenitride layer for the hard mask.

Then, although not shown, a series of known following processesincluding a LDD region forming process, a gate spacer forming process,and a junction region forming process are sequentially carried out toaccomplish the manufacturing of the semiconductor device having themetal gate according to the present invention.

In the above-mentioned embodiment of the present invention, on the otherhand, although the method of manufacturing the semiconductor devicehaving the metal gate with a plan gate structure has been described, thepresent invention may be applied for forming a metal gate with a recessgate structure.

Specifically, FIGS. 3A to 3E are sectional views illustrating a methodof manufacturing a semiconductor device having a metal gate according toanother embodiment of the present invention, in which the semiconductordevice is shown step by step. The same reference numerals are used toindicate the same elements as those of FIGS. 2A to 2D. Hereinafter, themethod of manufacturing the semiconductor device according to anotherembodiment of the present invention will be described.

Referring to FIG. 3A, a semiconductor substrate 21 is prepared, whichhas a isolation layer 22 defining an active region. Next, after anoxidation layer and a polysilicon layer are sequentially formed on thesemiconductor substrate including the isolation layer 22 although notshown, the polysilicon layer and the oxidation layer are etched toexpose the gate region in the active region of the semiconductorsubstrate.

Next, after the exposed gate region in the active region of thesemiconductor substrate is etched so that a recess 32 is formed, thepolysilicon layer and the oxidation layer used as an etching barrier areremoved. Then, well ion injection and channel ion injection aresequentially carried out, so that a gate insulation layer 23 is formedin the active region on the surface of the semiconductor substrate.

Referring to FIG. 3B, a polysilicon layer 24 is formed on thesemiconductor substrate including the gate insulation layer 23 so as tofill up the recess 32, and then a first tungsten silicide layer 25,which is a first metal silicide layer, is formed on the polysiliconlayer 24. Next, a tungsten nitride layer 26 is formed as a metal nitridelayer on the first tungsten silicide layer 25. Finally, a tungsten layer27 is formed as a metal layer on the tungsten nitride layer 26. Here,the first tungsten silicide layer 25 has a thickness of 200˜400 Å, thetungsten nitride layer 26 has a thickness of 50˜100 Å, and the tungstenlayer 27 has a thickness of 300˜400 Å.

Referring to FIG. 3C, the tungsten layer 27 and the tungsten nitridelayer 26 are etched according to a known process. At this time, theetching of the tungsten layer 27 and the tungsten nitride layer 26 iscarried out in the same manner as that of the aforementioned embodimentof the present invention, so that the etched tungsten layer 27 and thetungsten nitride layer 26 have a narrower width than that of a desiredgate.

Referring to FIG. 3D, a second tungsten silicide layer 28 is formed as asecond silicide layer on the first tungsten silicide layer 25 includingthe etched tungsten nitride layer 26 and the tungsten layer 27, so as tohave a thickness of 200˜400 Å.

Referring to FIG. 3E, a hard mask 29 is formed on the second tungstensilicide layer 28 according to the known process, which includes anitride layer defining a gate region and having a desired widthcorresponding to that of a gate. Then, the second silicide layer 28, thefirst silicide layer 25, the doped polysilicon layer 24 and the gateinsulation layer 23 are sequentially etched by using the hard mask 29 asan etching barrier. As a result, a metal gate, i.e. a tungsten gate 30is formed with a structure in which the tungsten layer 27 including thetungsten nitride layer 26 is enclosed with the first and second tungstensilicide layers 25 and 28.

Then, a gate re-oxidation process is carried out for the resultantsemiconductor substrate on which the tungsten gate 21 is formed, inorder to recover an etched damage. Next, a series of known succeedingprocesses of forming a LDD region, forming a gate spacer and forming ajunction region are sequentially performed to accomplish the method ofmanufacturing the semiconductor device having the metal gate accordingto the present invention.

Since the semiconductor device according to another embodiment of thepresent invention has a core type structure in which the tungstennitride layer 26 and the tungsten layer 27 are enclosed with the firstand second tungsten silicide layers 25 and 28, it is possible to preventthe abnormal phenomenon at the side of the tungsten layer 27 as well asto remove the instability of the interface, during the gate re-oxidationprocess. In addition, as the second tungsten silicide layer 28 depositedon the tungsten layer 27 plays the role of a buffer layer during theforming of the nitride layer for the hard mask, it is also possible toprevent the deterioration of the characteristics of the gate caused bythe stress of the nitride layer for the hard mask.

FIGS. 4A to 4B are sectional views illustrating a method ofmanufacturing a semiconductor device having a metal gate according tostill another embodiment of the present invention, in which thesemiconductor device is shown step by step. The same reference numeralsare used to indicate the same elements as those in FIGS. 2A to 2D.Hereinafter, the method of manufacturing the semiconductor deviceaccording to this embodiment of the present invention will be described.

The present embodiment relates to a method of manufacturing asemiconductor device having a metal gate with a step-gated asymmetryrecess structure. As shown in FIG. 4A, first, a semiconductor substrate21 is prepared, which has a isolation layer 22 defining an activeregion. Then, the semiconductor substrate has recesses 34 formed by adesired depth at both sides of an active region thereof along the lengthof the active region, so that a gate region is stepped. At this time,although not shown or described, a stacked layer of an oxidation layerand a polysilicon layer, or a stacked layer of an oxidation layer and anitride layer can be used as a mask for the recesses.

Next, a series of succeeding processes identical with those of theaforementioned embodiment of the present invention are performed for theresultant semiconductor substrate having the recesses 34 formed at bothsides of the active region thereof along the length of the activeregion, although not specifically shown or described. As shown in FIG.4B, a tungsten gate 30 is formed in the stepped active region on thesemiconductor substrate, which has a core type structure in which thetungsten nitride layer 26 and the tungsten layer 27 are enclosed withthe first and second tungsten silicide layers 25 and 28.

Finally, a series of known succeeding processes including a gatere-oxidation process, a LDD region forming process, a gate spacerforming process and a junction region forming process are sequentiallycarried out, so as to accomplish the manufacturing of the semiconductordevice having the metal gate.

Since the semiconductor device according to another embodiment of thepresent invention has the tungsten gate 30 formed with a core typestructure in which the tungsten layer 27 is enclosed with the first andsecond tungsten silicide layers 25 and 28, it is possible to prevent theabnormal oxidation phenomenon from occurring at the side of the tungstenlayer 27 during the gate oxidation process. In addition, it is possibleto remove the instability of the interface between the tungsten layer 27and the tungsten silicide layers 25 and 28. Moreover, the secondtungsten silicide layer 28 playing the role of buffering stress isdeposited on the tungsten layer 27, thereby effectively preventing thedeterioration of the characteristics of the gate caused by the stress ofthe nitride layer for the hard mask.

As described above, the present invention has the metal gate formed withthe core type structure in which the metal layer is completely enclosedwith the metal silicide layer, thereby preventing the abnormal oxidationof the tungsten and relieving the instability of the interface during asucceeding gate re-oxidation process.

According to the present invention, furthermore, the metal silicidelayer is disposed between the metal layer and the nitride layer for thehard mask as a buffer layer for buffering stress, thereby preventing thedeterioration of the characteristics of the metal gate resulting fromthe stress from the nitride layer of the hard mask and improving thecharacteristics of the device.

According to the present invention, in addition, since a metal layerwhich has greatly lower resistivity than the existing metal silicidelayer, is applied to the semiconductor device to form the gate, it ispossible to realize a high speed device, and to advantageously apply themetal layer for the manufacturing of large integrated device of 50 nmclass.

While a preferred embodiment of the present invention has been describedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

1. A method of manufacturing a semiconductor device, comprising thesteps of: preparing a semiconductor substrate having a isolation layerto define an active region; forming a gate insulation layer on thesemiconductor substrate; sequentially forming a polysilicon layer, afirst metal silicide layer, a metal nitride layer and a metal layer onthe gate insulation layer including the isolation layer; etching themetal layer and the metal nitride layer so that the metal layer and themetal nitride layer have a narrower width than that of a desired gate;forming a second metal silicide layer on the first metal silicide layerincluding the etched metal nitride layer and the metal layer; forming ahard mask on the second metal silicide layer so that the hard mask has adesired gate width; and etching the second metal silicide layer, thefirst metal silicide layer, the polysilicon layer and the gateinsulation layer by using the hard mask as an etching barrier, so as toform a metal gate with a structure in which the metal nitride and themetal layer are enclosed with the first and second metal silicidelayers.
 2. The method of manufacturing a semiconductor device as claimedin claim 1, further comprising a step of forming a junction region atboth sides of the metal gate on the semiconductor substrate, after thestep of forming the metal gate.
 3. The method of manufacturing asemiconductor device as claimed in claim 1, wherein the first and secondmetal silicide layers are formed with tungsten silicide, the metalnitride is formed with tungsten nitride, the metal layer is formed withtungsten, and the hard mask is formed with nitride.
 4. The method ofmanufacturing a semiconductor device as claimed in claim 1, wherein thefirst metal silicide layer has a thickness of 200˜400 Å.
 5. The methodof manufacturing a semiconductor device as claimed in claim 1, whereinthe metal nitride layer has a thickness of 50˜100 Å.
 6. The method ofmanufacturing a semiconductor device as claimed in claim 1, wherein themetal layer has a thickness of 300˜400 Å.
 7. The method of manufacturinga semiconductor device as claimed in claim 1, wherein the second metalsilicide layer has a thickness of 200˜400 Å.
 8. The method ofmanufacturing a semiconductor device as claimed in claim 1, furthercomprising a step of etching a gate region in the active region on thesemiconductor substrate so as to form recesses, after the step ofpreparing a semiconductor substrate and before forming a polysiliconlayer.
 9. The method of manufacturing a semiconductor device as claimedin claim 1, further comprising a step of recessing the both sides of theactive region on the semiconductor substrate so that a gate region isstepped in a lengthwise direction of the active region, after the stepof preparing a semiconductor substrate and before forming a polysiliconlayer.